Remove the `show` property from the modal plugin (#32279)

Remove the `show` property from the modal plugin (#32279)

Drop the support of show property from the modal plugin.
When creating the new modal instance in v5, the `show` property does not work anymore, so instead of fixing the behavior just removing it permanently to keep the consistency between all the plugins. (All other plugins require the `show()` method to be called on the instances to be shown)

Co-authored-by: XhmikosR <xhmikosr@gmail.com>